Channel estimation is a process by which a received wireless signal is adjusted by compensating for signal distortion caused by wireless channel fading. For example, fading causes signal strength to fluctuate rapidly due to multipath-time delay in a wireless communication system environment. Accordingly, to determine how the signal has been distorted, a reference signal known to both a transmitter and a receiver is transmitted through the channel so that the receiver can determine and compensate for wireless channel conditions affecting the reference signal.
A reference signal in an LTE wireless network is a signal having characteristics known to both a mobile station (e.g., a UE) and a base station (e.g., an eNB). Uplink reference signals are those that are produced by the UE for reception at the eNB. Downlink reference signals are those that are produced by the eNB for reception at the UE. Examples of downlink reference signals, explained in the following paragraphs, include a cell-specific reference signal (CRS), a UE-specific reference signal (DMRS), and a channel state information reference signal (CSI-RS).
In LTE release version no. 8 (LTE Rel-8) systems, many wireless communications facilities employ CRSs. For example, the following LTE components all use CRS-based channel information: physical downlink shared channel (PDSCH) demodulation, reference signal received power (RSRP) and reference signal received quality (RSRQ) (RSRP/RSRQ) measurements ranking candidate cells for handover and cell reselection decision, channel quality indicator (CQI) feedback, pre-coding matrix indicator (PMI) feedback, rank indication (RI) feedback, and other parameters.
In the LTE release version no. 10 (LTE Rel-10) standard, the fundamentally CRS-centric system was complemented by a UE-centric reference signal system. The UE-centric reference signals include DMRSs and CSI-RSs that a UE uses to acquire channel state information. These UE-centric reference signals fulfill a number of design goals, including reducing the reference signal overhead, providing interference measurability, reducing reference signal interference for coordinated multi-point (CoMP) transmission/reception (e.g., CoMP scenario 4 characterized by one common cell-ID shared among multiple cells), and other goals. Developing standard sets of pre-defined reference signals to address numerous channel and device deployment scenarios, however, is an ongoing challenge.